London’s tech start-up community is to lose two of its best-known investors to the west coast of America, as Index Ventures opens its first office in Silicon Valley.

Danny Rimer, who established Index’s London office in 2002 and made the company’s name with its lucrative early investment in Skype, and Mike Volpi, a former Cisco Systems executive who joined Index in 2009, will establish the European fund’s first permanent Californian presence by September.

As well as Skype, Index has been a backer of prominent British internet groups such as Last.fm, the music site, and Betfair, the gambling exchange which floated in London last year.

Mr Volpi told the Financial Times that the move did not represent any diminution of Index’s commitment to European tech firms, which the company has always championed as realistic challengers to those launched in the US.

“We have seen a slight increase in the percentage of opportunities we getpresented with in the US,” he said. “With a growing number of portfolio companies there, it seemed like a natural step.”

About $400m of the $1.3bn Index has invested in the past 15 years has been in the US, mostly in Silicon Valley. Alongside London’s Mayfair, Index also has offices in Geneva.

“The companies we serve today expect a dual presence,” Mr Volpi said. “It really is a must-have for a mainstream investment firm in this day and age to have solid feet in [the US and Europe].”

The move will see the number of active Index partners based in London fall from nine to seven.